1. Technical Field
This disclosure relates generally to a flow battery and, more particularly, to a flow battery having electrodes with a plurality of different pore sizes and/or different layers.
2. Background Information
A typical flow battery system includes a stack of flow battery cells, each having an ion-exchange membrane disposed between negative and positive electrodes. During operation, a catholyte solution flows through the positive electrode, and an anolyte solution flows through the negative electrode. The catholyte and anolyte solutions each electrochemically react in a reversible reduction-oxidation (“redox”) reaction. Ionic species are transported across the ion-exchange membrane during the reactions, and electrons are transported through an external circuit to complete the electrochemical reactions.
The negative and positive electrodes can be constructed from a carbon felt material. Such a carbon felt material typically has a plurality of interstices of substantially uniform size that promote uniform distribution of the electrolyte solution therethrough. Each electrode has a relatively large thickness (e.g., greater than 3.2 millimeters (mm), ˜125 thousandths of an inch (mil)) sized to reduce pressure drop across a length of the electrode, which length is substantially perpendicular to the thickness. Such a relatively large electrode thickness, however, can substantially increase resistance to ionic conduction across the thickness of the electrode. Electrodes with relatively large thicknesses, therefore, can increase voltage inefficiency of the flow battery cell due to the increased resistance to ionic conduction, especially when the flow battery cell is operated at relatively high current densities such as greater than 100 milli amps (mA) per square centimeter (cm2) (˜645 mA per square inch (in2)).
There is a need in the art, therefore, for a flow battery cell that can operate at relatively high current densities, without significantly increasing voltage inefficiency. Operating at relatively high current densities without excessive voltage losses can permit use of a smaller stack size and, therefore, a lower stack cost for a given power output.